Multi-module systems typically require sharing of frequency and phase reference signals for real-time calibration. In such systems, it is desirable to measure transmission characteristics between arbitrarily-selected ports of the modules. For example, in a phased-array radar system it is necessary to know the relative phase characteristics at the respective antennas in order to be able to direct a phased beam in a particular direction. In another example, multiple input/multiple output (MIMO) radar systems require referencing received signals to one another.
Under ideal conditions, measurement of reference signals is generally stratightforward. When transmission losses are high, however, signal leakage among module ports interferes with reference measurement. For example, when making multi-port measurements with a vector network analyzer (VNA) there is typically some signal leakage between the VNA's ports, which limits the dynamic range of the measurements. This problem is particularly pronounced in the case of a single RFIC, where the isolation is limited because of the small inter-port distances and the inherently-restricted isolation of the RFIC, the package, and the printed circuit board (PCB). Here, the likely limit for isolation is on the order of 50 dB, achieved between the most distantly-separated RFIC ports.
A known improvement to the above-described isolation problem is to use a separate shielded RFIC for each port. In this way, the signal transmitted to the device (or medium) under test (hereinafter denoted as “DUT”) has a significantly better isolation, and only the signal passing through the DUT reaches the other RFIC. Unfortunately, however, this introduces the problem of providing a phase reference to the mated RFIC. RFICs may have distinct synthesizers, so the phase of a signal from one RFIC downconverted within another RFIC cannot be directly measured—only comparative measurements can be made. This requires that a sample of the reference signal be provided to the receiving RFIC. The straightforward approach for providing the reference is to bring a sample of the transmitted signal to the receiving RFIC via a receiving port, and then measure the phase difference between the signal from the DUT and the reference signal from the transmitting RFIC. However, bringing a signal at the test frequency can contaminate the signal from the DUT, because the receiving RFIC has limited isolation. The problem could be lessened by weakening the reference signal, but doing so also reduces measurement accuracy because of the degraded signal-to-noise ratio of the reference.
Under the conditions and restrictions described above, it would be desirable to have methods for reducing or eliminating signal leakage; reducing or eliminating the affects of signal leakage on measurements; and making accurate measurements in spite of signal leakage. These goals are met by embodiments of the present invention.